1. Field of the Invention
The present invention relates to an electrolytic solution suitable for use in a 100 WV class electrolytic capacitor and to an electrolytic capacitor using this electrolytic solution.
2. Description of the Related Art
An electrolytic capacitor with an electrolytic solution has in a sealed case a structure comprising a positive electrode made of a foil of valve metal such as aluminum, tantalum and niobium with an oxide film as a dielectric on the surface, a negative electrode for current collection (an apparent negative electrode) and a separator holding the electrolytic solution between the positive electrode and the negative electrode, and a wound type or laminating type of electrolytic capacitor is widely used. In the capacitor, the electrolytic solution directly contacts the dielectric and functions as a true negative electrode, and the characteristic of the electrolytic capacitor largely depends on the kind of electrolytic solution.
An electrolytic capacitor used in the automobile industry requires a rating that enables it to function at a low temperature of −40° C. in addition to a voltage resistance of 50 WV or more. It also has to exhibit durability at high temperatures so it can be used within a high-temperature engine compartment etc. Further, it must show little deterioration in electrostatic capacitance and impedance characteristics after usage at 125° C. for a long period and must exhibit no structural weaknesses that can lead to problems such as liquid leakage. The applicant suggested an aluminum electrolytic capacitor that could meet these requirements in Patent Document 1 (JP 2001-223136 A). An electrolytic solution in this capacitor comprises a mixed solvent containing sulfolane, γ-butyrolactone and ethylene glycol, a quaternary imidazolinium salt or a quaternary pyrimidinium salt of carboxylic acid such as phthalic acid, boric acid amounting to 0.5 to 2.5% by mass of the total electrolytic solution, and mannitol amounting to 0.5 to 2.5% by mass of the total electrolytic solution. Especially, if 20 to 60% by mass of the mixed solvent is γ-butyrolactone, the dielectric loss of the capacitor decreases, while the high-temperature life characteristic and low-temperature characteristics improve.